Flow sensors are known in which a flow-representing output signal depends on changes that occur as a flow-actuated device passes one or more sensing electrodes. One type of this kind of flow sensor is designed for monitoring the flow of somewhat conductive liquid, notably tap water. My patents U.S. Pat. No. 4,333,354 issued Jun. 6, 1982, U.S. Pat. No. 4,399,696 issued Aug. 23, 1983 and U.S. Pat. No. 4,890,499 issued Jan. 2, 1990 are illustrative. Another type of this kind of flow sensor is useful for monitoring the flow of non-conductive fluids such as nonviscous hydrocarbon liquids and gases such as air. In those known flow sensors, there is a flow-actuated device that passes a capacitive electrode or multiple capacitive electrodes (considered further below). U.S. Pat. No. 4,164,866 issued Aug. 21, 1979, U.S. Pat. No. 4,324,144 issued Apr. 13, 1982 and U.S. Pat. No. 4,581,943 issued Apr. 15, 1986 are illustrative.
A so-called "in-line" form of these flow sensors involves a flow-actuated device that orbits inside a pipe that forms a passage, and in which the wall of the pipe supports the sensing electrode or electrodes. My patents U.S. Pat. No. 4,333,354 and U.S. Pat. No. 4,890,499 disclose in-line flow sensors in which the flow-actuated device is an orbital ball; and patents U.S. Pat. Nos. 4,164,866 and U.S. Pat. No. 4,324,144 disclose in-line flow sensors in which the flow actuated device is a vaned rotor.
A so-called "probe" form of flow sensor includes a supporting stem that projects through a pipe's wall and supports a flow-sensing assembly or multiple assemblies inside the pipe, each assembly including a flow-actuated device and one or more sensing electrodes. My patents U.S. Pat. Nos. 4,379,696 and 4,829,833 disclose probe-type flow sensors; and in those patents, each flow-actuated device is a vaned rotor; and each of those flow sensors is disclosed as being used in monitoring the flow of somewhat conductive liquid. In the probe type of flow-sensing apparatus, the flow-sensing assembly of the electrode(s) and the flow-actuated device, including the frame structure that maintains the parts assembled, has a practical constraint: its dimensions are limited so that the assembly can be inserted into the pipe via a hole of limited size.
In flow-sensing apparatus used for sensing the flow of non-conductive fluid, the response of the sensing electrode(s) and the connected circuit to the flow-actuated device depends on the capacitive effect of the flow-actuated device on the sensing electrode(s). The sensing electrode(s) and the sensing apparatus including its circuit connected to the sensing electrode(s) are here called "capacitive sensing electrodes" and "capacitive flow-sensing apparatus" to signify their capability of responding to capacitive effects. The sensing electrode or each sensing electrode acts as an electrode of a capacitor; it is differently affected by the different dielectric and/or loss properties of the flow-activated device and the fluid to be monitored. The sensing capacitive electrode or each of a pair of such sensing electrodes in a flow sensor is inherently quite small so that the capacitance identified with such a sensing electrode is typically 1.0 pf, perhaps 1.0 to 3.0 pf.
Changes that affect only the capacitive electrode determine response to the flow-actuated device; stray capacitance of any wiring or other conductive structure connected to a sensing electrode adds to the total capacitance but stray capacitance is not affected by the flow-actuated device. As the stray capacitance increases, the percentage of the flow-representing change in the total capacitance decreases until a point is reached where the changes are not readily detectable. For that reason, in practical apparatus exemplifying my '943 patent, stray capacitance of wiring from the sensing electrode to its circuit is minimized, as by using fine wire and by using maximum spacing between such wire and nearby grounded structure. Heretofore it has not been practical to make a two-rotor flow sensing probe (such as that shown in my '696 patent) having capacitive sensing electrodes because of the large amount of stray capacitance of necessary wiring to the lower sensing electrode 32. That wiring may be 15 inches long.